Apparatus and method for forming carbon protective layer

ABSTRACT

An apparatus and method for forming a carbon protective layer on a substrate using a plasma CVD method allows a more uniform in-plane distribution of the carbon protective layer thickness. The apparatus includes an annular anode that generates a plasma beam and a disk-shaped shield disposed between the anode and the substrate. The anode, the shield, and the substrate are concentrically arranged so that a straight line connecting the centers of the anode and the substrate is perpendicular to the deposition surface of the substrate where the carbon protective layer is to be formed. The center of the shield is also on the straight line.

BACKGROUND

Examples of carbon protective layers used for coating sliding-resistantmembers or friction-resistant members include diamond-like carbon (DLC)films and graphite carbon films. Carbon protective layers are formed onmagnetic recording layers of magnetic recording media to serve assliding-resistant members to protect the magnetic recording layers fromdamage or corrosion induced by sliding and contact with magnetic heads.In recent years, DLC films formed by a plasma CVD method have been used.This is because a DLC film formed by the plasma CVD method is denser andharder than a film formed by a sputtering method. This feature isapparently due to the fact that the DLC film formed by the plasma CVDmethod is formed by hydrocarbon radicals, and a tetrahedral structurewith a high three-dimensional rigidity is easily formed via hydrogen.

The increase in recording density is a requirement applied to magneticrecording media, and reducing the thickness of protective film anddecreasing the distance between a head element and a magnetic recordinglayer is an effective means for meeting this requirement. When the filmthickness is reduced, it is important to have a uniform in-planedistribution of the film thickness. Thus, even where the in-planeaverage value of film thickness is sufficient to obtain thepredetermined electromagnetic conversion characteristic and thethickness is sufficient to ensure reliability, such as corrosionresistance, where the in-plane distribution of film thickness isnonuniform, the predetermined electromagnetic conversion characteristiccannot be obtained in thick places, whereas the reliability such ascorrosion resistance cannot be ensured in thin places.

A method of disposing an appropriate shield within a plasma CVD chamberwhen a DLC film is formed by a plasma CVD method with the object ofobtaining a uniform in-plane distribution of protective film thicknesshas been investigated. See for example Japanese Patent ApplicationsLaid-open Nos. 2001-148118 and 2003-30823. With the usual plasma CVDmethod, the film thickness in the central zone of a substrate for filmformation tends to increase because plasma irradiating the substrate isconcentrated in the central zone. Disposing a shield in the vicinity ofthe central zone can make the film thickness more uniform. See, forexample, paragraph 0010 of Japanese Patent Application Laid-open No.2003-30823.

Further improvements, however, are needed to obtain a more uniform filmthickness. Accordingly, there still remains a need for forming a carbonprotective layer using a plasma CVD method, which makes it possible tobring the in-plane distribution of film thickness close to a uniformdistribution.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus and a method for forming acarbon protective layer on a substrate, such as for a magnetic recordingmedium, by a plasma CVD method.

One aspect of the present invention is an apparatus for forming a carbonprotective layer on a substrate by a plasma CVD method. The apparatusincludes an annular-shaped plasma beam generation source and adisk-shaped shield. The annular-shaped plasma beam generation source isconfigured to be spaced from and disposed parallel to a depositionsurface of the substrate, where the carbon protective layer is to beformed. The disk-shaped shield is spaced from the annular-shaped plasmabeam generation source and configured to be positioned between theannular-shaped plasma beam generation source and the substrate. Thedisk-shaped shield is concentrically arranged with the annular-shapedplasma beam generation source and the deposition surface of thesubstrate so that a straight line connecting a center of the annularplasma beam generation source and a center of the deposition surface ofthe substrate is perpendicular to the deposition surface of thesubstrate.

The disk-shaped shield has a radius equal to an overlap radius A, and adistance x between the disk-shaped shield and the annular-shaped plasmabeam generation source satisfies the equation x=[{R/(R+A)}+B]·L, where Ris a radius of the plasma beam generation source, L is a distancebetween the annular-shaped plasma beam generation source and thedeposition surface of the substrate, and B is a range of −0.1 or more to0.1 or less.

Another aspect of the present invention is a method of forming a carbonprotective layer on a substrate by a plasma CVD method. The methodincludes disposing the deposition surface of the substrate, where thecarbon protective layer is to be formed, parallel to and spaced from theannular-shaped plasma beam generation source, and disposing thedisk-shaped shield spaced from the annular-shaped plasma beam generationsource between the annular-shaped plasma beam generation source and thesubstrate. The disk-shaped shield is concentrically arranged with theannular-shaped plasma beam generation source and the deposition surfaceof the substrate so that a straight line connecting a center of theannular plasma beam generation source and a center of the depositionsurface of the substrate is perpendicular to the deposition surface ofthe substrate.

Another aspect of the present invention is a magnetic recording mediumhaving a carbon protective layer formed by the apparatus or the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a sectional view of an apparatus forforming a carbon protective layer according to the present invention.

FIG. 2 explains the position and size of a disk-shaped shield of theapparatus of FIG. 1.

FIG. 3 explains the overlap radius A.

FIG. 4 illustrates the results obtained in Examples 1 to 3 andComparative Examples 1 and 2.

DETAILED DESCRIPTION

Referring to FIG. 1, an apparatus 100 for forming a carbon protectivelayer using a plasma CVD method has an annular anode 101 that generatesa plasma beam and a disk-shaped shield 102 disposed between the annularanode 101 and a substrate 103 for forming a film. The annular anode 101and a deposition surface or region of the substrate, where the carbonprotective film is to be formed, are parallel to each other andconcentrically arranged so that a straight line connecting the centersof the anode and the deposition surface is perpendicular to thedeposition surface. The center of the disk-shaped shield 102 is on thestraight line and also concentrically arranged with the anode 101 andthe substrate 103.

Benefits can be achieved in the apparatus 100 that features acombination of the annular anode 101 and the disk-shaped shield 102. Byproviding the disk-shaped shield 102, the concentration of plasma in thecentral zone of the substrate can be avoided and the in-planedistribution of film thickness can be made more uniform. By using theannular-shaped anode serving as a plasma beam generation source, theplasma density in the central zone can be decreased so that even moreuniform in-plane distribution can be realized.

Further, by forming a magnetic recording medium comprising thesubstrate, which can be the base material thereof, and a magneticrecording layer disposed on the base material, it is possible to form acarbon protective layer on a magnetic recording layer with the apparatus100. The present invention is intended to cover the magnetic recordingmedium.

FIG. 2 explains the position and size of the disk-shaped shield. Thedistance between the annular anode 101 and the deposition location ofthe substrate 103 is denoted by L, the distance between the annularanode 101 and the disk-shaped shield 102 is denoted by x, and the radiusof the annular anode 101 is denoted by R. According to the resultsobtained in the below-described examples, the uniformity of in-planedistribution of film thickness can be improved when the distance x isobtained from the following Equation: x=[{R/(R+A)}+B]·L, while makingthe radius of the disk-shaped shield 102 equal to A, which represents aradius at the position on the substrate that is overlapped with theplasma beam (referred to hereinbelow as “overlap radius”) when nodisk-shaped shield is provided, and B being a range (tolerance) of −0.1or more to 0.1 or less.

The overlap radius is calculated in the following manner. First, acarbon protective layer is formed without providing a disk-shapedshield, and a thickness of the carbon protective layer on the substratefor film formation is measured. The film thickness distribution in theradial direction is plotted, as shown in FIG. 3, the difference in filmthickness between the radial position of the outer circumferentialportion of the central hole (referred to hereinbelow as the “innermostcircumference”; for example, with a radius of 12.5 mm) and the radialposition of the outer radial portion of the substrate (referred tohereinbelow as the “outermost circumference”) is taken as 100%, and theradial position in which the film thickness has changed by 30% withrespect to that at the innermost circumference is taken as A. Thebroken-line portion of the film thickness distribution indicates aportion of the central hole. On the inside of the overlap radius A, theplasma beam overlaps as shown in FIG. 2, and the film thicknessincreases. Therefore, the film thickness reaches a maximum on theinnermost circumference and a minimum on the outermost circumference,and usually the radial position in which the film thickness decreases by30% with respect to that at the innermost circumference is taken as A.

With the apparatus 100 having the above configuration, the in-planedistribution of film thickness can be made more uniform, withoutdecreasing the film formation rate. Examples will be described below.The results obtained in Examples 1 to 3 and Comparative Examples 1 and 2are shown in FIG. 4.

In Example 1, in a hollow-cathode plasma CVD apparatus, the distance Lbetween the anode 101 and the deposition location (i.e., surface) of thesubstrate 103 was set at 160 mm, the distance x between the anode 101and the 102 shield was set at 115 mm, and the radius R of the anode 101was set at 45 mm. The shield was made of stainless steel and had athickness of 2 mm, and the electric potential thereof was a groundpotential. The substrate 103 was made of aluminum and had a diameter of95 mm and a thickness of 1.27 mm.

When a DLC film was formed on a magnetic recording medium formed toobtain a magnetic recording layer using the substrate 103, because theoverlap radius A was 17.5 mm, the shield radius was set at 17.5 mm.Here, argon (30 sccm) was used as a discharge gas, and acetylene (30sccm) was used as a raw material gas. Calculating the distance x usingthe Equation, the following value is obtained: x=(0.720±0.1) 160(mm)=115±16 (mm). In the Example 1, x was set at 115 mm, thus satisfyingthe equation.

Under the above conditions, the film formation time was adjusted and aDLC film with a thickness of 2.5 nm in the intermediate circumferentialportion was obtained. The “inner circumferential portion” means a regionon the inner circumferential side in a data recording portion of adisk-shaped magnetic recording medium and generally indicates a regionwith a radius of about 20 mm in the case of a rated 3.5 inch substrate(substrate with a diameter of 95 mm). The “outer circumferentialportion” means a region on the outer circumferential side in a datarecording portion of a disk-shaped magnetic recording medium andgenerally indicates a region with a radius of about 40 mm in the case ofa rated 3.5 inch substrate. The “intermediate circumferential portion”indicates a region between the “inner circumferential portion” and the“outer circumferential portion” and generally indicates a region with aradius of about 30 mm in the case of a rated 3.5 inch substrate.

The in-plane distribution of film thickness was measured with an opticalsurface inspection device. The film thickness was 2.5 nm in theintermediate circumferential portion, 2.5 nm in the innercircumferential portion, and 2.4 nm in the outer circumferentialportion, and the radial distribution of film thickness (the ratio of thedifference between the maximum value and minimum value divided by thevalue in the intermediate circumferential portion) was less than 6%,which is good.

In Example 2, a DLC film was formed under the same conditions as inExample 1, except that the distance x between the anode and the shieldwas set at 105 mm. The set distance x is still within the range ofx=115±16 (mm), thus satisfying the Equation. The in-plane distributionof film thickness was measured with an optical surface inspectiondevice. The film thickness was 2.5 nm in the intermediatecircumferential portion, 2.53 nm in the inner circumferential portion,and 2.4 nm in the outer circumferential portion, and the radialdistribution of film thickness was still less than 6%.

In Example 3, a DLC film was formed under the same conditions as inExample 1, except that the distance x between the anode and the shieldwas set at 125 mm. The set distance x is still within the range ofx=115±16 (mm), thus satisfying the Equation. The in-plane distributionof film thickness was measured with an optical surface inspectiondevice. The film thickness was 2.5 nm in the intermediatecircumferential portion, 2.48 nm in the inner circumferential portion,and 2.4 nm in the outer circumferential portion, and the radialdistribution of film thickness was still less than 6%.

In Comparative Example 1, a DLC film was formed under the sameconditions as in Example 1, except that the distance x between the anodeand the shield was set at 95 mm. The set distance x was outside therange of x=115±16 (mm), thus not satisfying the Equation. The in-planedistribution of film thickness was measured with an optical surfaceinspection device. The film thickness was 2.5 nm in the intermediatecircumferential portion, 2.7 nm in the inner circumferential portion,and 2.4 nm in the outer circumferential portion, and the radialdistribution of film thickness was more than 6%, which is worse than inExamples 1 to 3. This is apparently because the disk-shaped shield wastoo far from the substrate 103, and shielding of the plasma beam thattended to concentrate in the central zone of the substrate becameinsufficient.

In Comparative Example 2, a DLC film was formed under the sameconditions as in Example 1, except that the distance x between the anodeand the shield was set at 135 mm. The set distance x was outside therange of x=115±16 (mm), thus not satisfying the Equation. The in-planedistribution of film thickness was measured with an optical surfaceinspection device. The film thickness was 2.5 nm in the intermediatecircumferential portion, 2.3 nm in the inner circumferential portion,and 2.4 nm in the outer circumferential portion, and the radialdistribution of film thickness was more than 6%, which is worse than inExamples 1 to 3. This is apparently because the disk-shaped shield wastoo close to the substrate 103, and shielding of the plasma beam thattended to concentrate in the central zone of the substrate becameexcessive.

In Comparative Example 3, a DLC film was formed under the sameconditions as in Example 1, except that the radius of the disk-shapedshield was set at 7.5 mm. The distance x calculated using the Equationwas x=(0.857±0.1) 160 (mm)=137±16 (mm). Since the distance x was set at115 mm, the Equation was not satisfied. The in-plane distribution offilm thickness was measured with an optical surface inspection device.The film thickness was 2.5 nm in the intermediate circumferentialportion, 2.7 nm in the inner circumferential portion, and 2.4 nm in theouter circumferential portion, and the radial distribution of filmthickness was more than 6%, which is worse than in Examples 1 to 3. Thisis apparently because the disk-shaped shield was too small, andshielding of the plasma beam that tended to concentrate in the centralzone of the substrate became insufficient.

In Comparative Example 4, a DLC film was formed under the sameconditions as in Example 1, except that the radius of the disk-shapedshield was set at 27.5 mm. The distance x calculated using the Equation1 was x=(0.621±0.1) 160 (mm)=99.3±16 (mm). Since the distance x was setat 115 mm, the Equation was satisfied. The in-plane distribution of filmthickness was measured with an optical surface inspection device. Thefilm thickness was 2.5 nm in the intermediate circumferential portion,2.48 nm in the inner circumferential portion, and 2.4 nm in the outercircumferential portion, and the radial distribution of film thicknesswas less than 6%, which is good. But the film formation time necessaryto form a film with a thickness of 2.5 nm was 1.9 times greater that inExample 1, thus decreasing productivity. This is apparently because,when the disk-shaped shield is too large, although the uniformity of thefilm thickness distribution in the radial direction is improved, theshielding effect decreased the overall deposition rate.

While the present invention has been particularly shown and describedwith reference to preferred embodiments, it will be understood by thoseskilled in the art that the foregoing and other changes in form anddetails can be made therein without departing from the spirit and scopeof the present invention. All modifications and equivalents attainableby one versed in the art from the present disclosure within the scopeand spirit of the present invention are to be included as furtherembodiments of the present invention. The scope of the present inventionaccordingly is to be defined as set forth in the appended claims.

This application is based on and claims priority to Japanese PatentApplication 2007-242541 filed on 19 Sep. 2007. The disclosure of thepriority application in its entirety, including the drawings, claims,and the specification thereof, is incorporated herein by reference.

1. An apparatus for forming a carbon protective layer on a substrate bya plasma CVD method, the apparatus comprising: an annular-shaped plasmabeam generation source configured to be spaced from and disposedparallel to a deposition surface of the substrate, where the carbonprotective layer is to be formed; and a disk-shaped shield spaced fromthe annular-shaped plasma beam generation source and configured to bepositioned between the annular-shaped plasma beam generation source andthe substrate, wherein the disk-shaped shield is concentrically arrangedwith the annular-shaped plasma beam generation source and the depositionsurface of the substrate so that a straight line connecting a center ofthe annular-shaped plasma beam generation source and a center of thedeposition surface of the substrate is perpendicular to the depositionsurface of the substrate.
 2. The apparatus for forming a carbonprotective layer according to claim 1, wherein the disk-shaped shieldhas a radius equal to an overlap radius A, and a distance x between thedisk-shaped shield and the annular-shaped plasma beam generation sourcesatisfies the equation x=[{R/(R+A)}+B]·L, where R is a radius of theannular-shaped plasma beam generation source, L is a distance betweenthe annular-shaped plasma beam generation source and the depositionsurface of the substrate, and B is a range of −0.1 or more to 0.1 orless.
 3. A magnetic recording medium having a carbon protective layerformed according to the apparatus of claim
 1. 4. A magnetic recordingmedium having a carbon protective layer formed according to theapparatus of claim
 2. 5. A method of forming a carbon protective layeron a substrate by a plasma CVD method, the method comprising the stepsof: disposing the substrate so that a deposition surface of thesubstrate, where the carbon protective layer is to be formed, isparallel to and spaced from an annular-shaped plasma beam generationsource; and disposing a disk-shaped shield spaced from the annularplasma beam generation source between the annular-shaped plasma beamgeneration source and the substrate, wherein the disk-shaped shield isconcentrically arranged with the annular-shaped plasma beam generationsource and the deposition surface of the substrate so that a straightline connecting a center of the annular-shaped plasma beam generationsource and a center of the deposition surface of the substrate isperpendicular to the deposition surface of the substrate.
 6. The methodaccording to claim 5, wherein the disk-shaped shield has a radius equalto an overlap radius A, and a distance x between the disk-shaped shieldand the annular-shaped plasma beam generation source satisfies theequation x=[{R/(R+A)}+B]·L, where R is a radius of the annular-shapedplasma beam generation source, L is a distance between theannular-shaped plasma beam generation source and the deposition surfaceof the substrate, and B is a range of −0.1 or more to 0.1 or less.
 7. Amagnetic recording medium having a carbon protective layer formedaccording to the method of claim
 5. 8. A magnetic recording mediumhaving a carbon protective layer formed according to the method of claim6.